PET (Positron Emission Tomography) equipment reconstructs sectional images of a subject only upon detection of positron, i.e., two or more gamma rays generated in annihilation of the positron and, detection of the gamma rays simultaneously with two or more detectors.
The PET equipment of this type doses a subject with a radioactive drug, and thereafter determines accumulation of the drug in a target tissue temporally. As a result, various body functions may be determined quantitatively. Consequently, an image that the PET equipment obtains has functional information.
Here, in techniques to simultaneously detect gamma rays, i.e. to perform coincidence of gamma rays, a 3D-PET that detects gamma rays three-dimensionally has been recently used besides a 2D-PET that detects gamma rays two-dimensionally. In such 3D-PET, each of the detectors is arranged close to the subject at a large solid angle, which results in enhanced detection efficiency of gamma rays and significantly improved system sensitivity.
For performing coincidence of gamma rays, each of gamma rays is inputted into a coincidence circuit to determine on whether or not a time lag of the inputted gamma rays is kept within a given time window. In an actual coincidence circuit, gamma rays are typically considered “coincident” that are detected in an extremely short time window of around 4 ns to 20 ns (ns=10−9 s). Consequently, there arises a possibility of performing coincidence of each one of gamma rays generated at two different points. This is called “random, coincidence count.” FIG. 12(a) is a schematic view exemplarily showing a state of the random coincidence. On the other hand, where coincidence is performed after one or both of a pair of gamma rays causes Compton scattering within the subject, the coincidence is called “scatter coincidence count.” FIG. 12(b) is a schematic view exemplarily showing a state of the scatter coincidence. A portion shown in the detector in FIG. 12 by hatching illustrates a detector that performed coincidence. Where coincidence of both a pair of gamma rays is normally performed, the coincidence is called “true coincidence count” (see, for example, Patent Literatures 1, 2).
In order to enhance image quality of the PET, it needs to increase the number of true coincidence counts (T) to enhance statistical accuracy, and also to suppress noise amplification in various corrections. As for an approach to enhance statistical accuracy, a dosage of radiopharmaceutical may be increased or a data acquisition time to perform coincidence for data acquisition may be extended. However, even if the true coincidence count (T) doubles by increasing dosage by twice, the random coincidence count (R) will increase by 4 times, which results in increased noise amplification in correction of the random coincidence count. In addition, scattered coincidence count (S) is to be included that varies depending on a size of the subject and distribution of radioactivity. Here, noise equivalent count (NEC: Noise Equivalent Count) is used as an index of simple evaluation of the PET image quality from the counts T, S, and R (see, for example, Non-Patent Literature 1.)
Where the random coincidence count is measured and corrected, the noise equivalent count NEC is given by the following equation (1) using a circuit that is a combination of the coincidence circuit with a delay circuit (delayed coincidence circuit). Moreover, where the random coincidence count is estimated and corrected from a single counting rate, the noise equivalent count NEC is given by the following equation (2).NEC=T2/(T+S+2×f×R)  (1)NEC=T2/(T+S+f×R)  (2)
Where, f in the foregoing equations (1) and (2) is a ratio of the subject to a gantry with a gamma-ray detector being embedded therein. Specifically, the rate is a rate of the subject to an aperture diameter of the gantry (i.e., an aperture diameter of the gamma-ray detector.)
[Patent Literature 1]    Japanese Patent Publication No. 2000-28727 (page 3, FIG. 5)
[Patent Literature 2]    Japanese Patent Publication No. H07-113873 (page 1-7 and 9-11, FIGS. 2, 5, 7, 8 and 13)
[Non-Patent Literature 3]    Keiichi Matsumoto and other 5 persons: “Comparison of Noise Equivalent Count Rate and Image Quality of Two-dimensional and Three-dimensional PET Scans”, Japanese Journal of Radiological Technology, Vol. 62, No. 8, P1111-1118 (2006.8)